The invention relates to a device for effecting a modification in a stream of transmission blocks containing information, provided with
detection means for detecting a transmission block to be modified and for generating a detection signal,
memory means for generating, in response to the detection signal, the modification to be applied, and
combining means for combining the stream of transmission blocks and the modification to be applied, provided with a first input for receiving the stream of transmission blocks, with a second input for receiving the modification to be applied and with an output for generating a modified stream of transmission cells.
A device of this type of known from I.E.E.E. Communications Magazine No. 4, April 1988, New York, USA, "The QPSX Man", R. M. Newman, Z. L. Budrikis, J. L. Hullet, pp. 20-28, in particular FIG. 2. The device described therein comprises an Access Unit (AU) which forms the detection means for detecting the transmission block to be modified, and which forms the memory means for generating the modification to be applied. An OR gate forms the combining means for combining the stream of transmission blocks and the modification to be applied. The OR gate is provided with the first input for receiving the stream of transmission blocks and the second input for receiving the modification to be applied. As soon as the header of the transmission block to be modified is detected by the AU, the AU generates the modification to be applied in the form of bits which are fed to the second input of the OR gate, the original information contents of the transmission block to be modified being fed at the same time to the first input of the OR gate. If the original information contents have the value zero, (i.e. all bits thereof have the logical value zero), the new information contents are formed by the modification to be applied. If the original information contents have a value not equal to zero (i.e. at least one of the bits thereof has the logical value one), the new information contents are formed by the logical bitwise addition by the OR gate of the original information contents and the modification to be applied. In this last case, either the original information contents should be known to the AU in advance, or, after detection by the AU, a calculation should be carried out which enables the AU to generate the modification, with which, by logical bitwise addition by the OR gate, the desired new information contents are obtained.
The known device has the drawback that, if the original information contents of the transmission blocks to be modified have a value not equal to zero, there should either be prior knowledge located at the AU or a calculation should be carried out by the AU, which takes time and which limits the maximum clock rate of the stream of transmission blocks.